The present invention relates to a focus error (degree of out-of-focus) detecting method and an optical head using it, and more particularly to a focus error detecting method which can easily adjust the position of an optical component and an optical head which can be used for an optical information processing device such as an optical disk device, an optical card device and an optical tape device.
An optical disk storage device has been developed as an information storage device which is adapted to be able to record, reproduce or erase data on a rotary recording medium with high density.
Most of the focus error detecting techniques adopted for the optical head in such an optical disk device are based on the fact that the shape or light intensity distribution of the light reflected from a disk varies in accordance with the focus error of the light. In this case, the light reflected from the disk is received by a multi-division photo-detector, and unbalance in the DC-like output signals is used as a signal for detecting the focus error.
For example, the astigmatical technique is disclosed in JP-A-53-19806 (first related art). If the beam reflected from a disk is given astigmatical by an astigmatical element such as a cylinder lens, it form two focal lines perpendicular to each other to provide a circular shape at the minimum confusion circle position in its rough center. Therefore, the reflected beam is received by the four-divided photo-detector located at the minimum confusion circle position. The shape of the reflected beam formed on the detection surface of the four-divided photo-detector is approximately circular if the disk is at the focal point. If the disk is displaced from the focal point, the shape of the reflected beam lines perpendicular to each other according to the direction of displacement. Thus, the DC-like output signals from photo-detector elements located at diagonal line positions of the four-divided photo-detector are added, and a difference between two DC-like added signals is taken to provide an out-of focus signal.
On the other hand, although in JP-A-1-303632 (second related art), changes in the shape and light intensity distribution of the light reflected from the disk are detected by the multi-division photo-detector, a focus error is not detected by detecting unbalance in the DC-like output signals resulting therefrom, but detected using a diffraction grating which forms light spots at different positions in a focus depth direction.
In an embodiment described in this JP-A-1-303632, the diffraction grating constitutes a part of a plurality of coaxial circle grooves in which the interval between the grating grooves are gradually increased or decreased, and gives aberration of longitudinal shift of an image point (aberration of focus) to a + first-order beam and an - first-order beam. Also, using an area where the center of the concentric circle grooves is eccentric from the main axis of a main beam, the diffraction grating emits the + first-order beam and the - first-order beam in opposite directions with respect to the main axis of the main beam. Thus, the + first-order beam and - first-order beam are focused by an objective lens as two side spots at different positions in the focus depth direction with respect to the main beam. The amounts of reflection light of the two side spots are modulated by signals recorded on an optical disk. The modulation degrees thus obtained are detected by a photo-detecting element and an envelope detecting circuit. The modulation degrees due to the two side spots vary in the focus error of the optical disk so that a difference therebetween is calculated to provide a focus error detecting signal.
The above related arts involves the following problems.
In the first related art, the target point in the focus error detecting optical system must be set so that a disk position where a data signal or the like is largest, or the amount of reflected light is maximum must be detected by an adjustment measuring system which is separately provided from a focusing system. And positions of the four-divided photo-detector and a detecting lens must be adjusted so that the focus error detecting signal is at a zero level because at the above disk position, each of the detecting elements of the four-divided photodetector receives an equal amount of light reflected from the disk and so produces a D.C. signal at the same level. Therefore, it takes a long time to assemble the optical head and adjust it; the optical head thus fabricated is very expensive. Further, the optical head must be provided with a mechanism for adjusting optical components with high accuracy so that it is difficult to realize a small-sized optical head. Moreover, if the positions of attaching the optical components are deviated from their normal position owing to a temperature change, the position of the reflected light on the detecting surface of the four-divided photo-detector will also vary. Then, DC-like unbalance occurs among the outputs from the respective detecting elements of the four-divided photo-detector so that the disk position where the focus error detecting signal is at a zero level does not align with the target position for focusing. As a result, the focus error detecting signal will involve an offset (detecting error).
Further, in the waveguide type optical head which is considered to be one type of the optical head to be realized in the feature, the beam which can travel within the waveguide has one-dimensional distribution. Therefore, few conventional focus error detecting systems using a change in the beam shape can be applied to the waveguide type optical head. Furthermore, in the waveguide type optical head, lens elements and photo-detecting elements are integrally fabricated on a waveguide substrate through a crystal growth process so that the positions of these elements cannot be shifted for focusing adjustment.
On the other hand, the above second related art can solve the above problems involved in the first related art. However, this second related art does not sufficiently study an arrangement of pits which permits a focus error signal to be obtained in a stabilized manner and the structure of a diffraction grating for defining the shape of side spots. Therefore, the operation for focusing control becomes unstable because of reduction in the modulation ratio due to a change in the relative position between the side spots and the pits in the radial direction of the optical head. Generally, automatic focusing control should be made prior to automatic tracking control, and must not be affected by the position change in the radial direction of the optical disk. This will be explained in detail with reference to FIGS. 27A, 27B and 27C.
In FIGS. 27A and 27B, 200a is a main spot on the surface of an optical disk; 200b and 200c are sub-spots on the optical disk surface; 201 is a track; and 202 are pits recorded on the track 201. It is assumed in both FIGS. 27A and 27B that the position of the optical disk in the direction of focal depth is deviated from the focal point of the main spot and the deviation is smallest at the position of the sub-spot 200c having the smallest area (i.e. the optical disk is located in the neighborhood of the focal point of the sub-spot 200c).
FIG. 27A shows the case where the above three spots travel rightly along the center of the track 201. In this case, the reflection light of the sub-spot 200c is modulated by the pit 202. Although not shown, if the optical disk is deviated in the direction opposite to the above case, and hence the area of the sub-spot is smallest, the reflection light of the sub-spot 200b will be modulated by the pit 202. Thus, if the above three spots travel rightly along the center of the track 201, the resulting focus error detecting signal will have the curve indicated by a solid line 203 in FIG. 27C.
On the other hand, FIG. 27B shows the case where the above three spots travel along the line deviated from the center of the track 201. In this case, the reflection light of the sub-spot 200c is not so greatly modulated by the pit 202, but the degree of modulation will be decreased to e.g. half or less in the case of FIG. 27A. Although not shown, also in the case the optical disk is deviated in the direction opposite to the above case, and hence the area of the sub-spot is smallest (i.e. the optical disk is located in the neighborhood of the sub-spot 200b), the degree of modulation of the reflection light of the sub-spot 200b will be decreased to half or less in the case of FIG. 27A. Thus, if the three spots travel along the line deviated from the center of the track 201 as in FIG. 27B as indicated by a broken line 204 in FIG. 27C the peak of the resulting focus error detecting signal will be reduced to half or less as compared with the curve 203, and so the detecting sensitivity in the neighborhood of the focal point will be reduced to half or less. Namely, if in this related art, the optical disk is deviated from the focal point of the main spot, it will approach the focal point of the sub-spot 200b or 200c and so the sub-spot will be confined into a small area. Then, the reflection light of the confined sub-spot, if the confined spot is deviated from the center of the track 201, will not be sufficiently modulated by the pit 202.
If the detection sensitivity is low in the pull-in operation into the automatic focusing control, driving force of causing an objective lens to follow the deviation of the optical disk will run short. As a result, the movement of the objective lens becomes slow so that the pull-in operation into the focusing control may end in failure. In this way, the second related art has a defect of making the pull-in operation into the focusing control unstable.
On the other hand, in automatic tracking control, a target can be automatically set to follow a track. And in order that in the automatic tracking control, no offset theoretically occur even when the positions of optical elements vary, a sample serve system using pre-wobbled pits has been proposed. In this sample serve system, an out-of-track is detected on the basis of the fact that when a main spot passes at least a pair of right and left pre-wobbled pits arranged with an equal amount of deviation from the track center, the modulation levels of the beams reflected from the right and left pre-wobbled pits vary in accordance with the degree of out-of-track (tracking error) of the main spot. The reflected beam is received by a photo-detector to detect the difference between the above modulation levels, thus providing an tracking error detecting signal. In this method, if the main spot travels along the track center, the tracking error detecting signal automatically becomes a zero level. This makes it unnecessary to adjust the positions of optical elements in order to set the target to follow the track. Further, the photo-detector has only to receive the entire amount of light of the reflected beam, and a change in the positions where optical components are attached will not provide any detecting error to the tracking error. However, in order to realize a small-sized optical head at low cost, both focus error and tracking error are detected in the same optical system and so both photo-detecting elements for detecting the focus error and the tracking error are incorporated in the same photo-detector package. Therefore, in assembling the optical head, it is not necessary to adjust the positions of the optical components for track-following target setting, but still necessary to adjust them at high accuracy for focusing. Thus, the above technique does not effect the merit of the sample servo system.